1. Field of the Invention
This invention relates to bicycle frames. It is particularly related to light weight, very strong bicycle frames made of composite materials.
2. Discussion of Prior Art
Traditional Metal Bicycle Frame
Bicycle frames have traditionally been constructed of assembled metal tubes. A variety of methods have been used to connect the metal tubes. The most common methods have included welding, soldering or brazing. It has also been common to use preformed metal lugs as connectors at the junctions of the metal tubes. The preformed lugs reinforce, or add strength and stiffness to the welded, soldered, or brazed joint.
Bicycle Frames Utilizing Advanced Composite Tubes
Some modern bicycle frames utilize composite materials to increase the strength and stiffness of the frames, while reducing frame weight, relative. to traditional metal frames. Composite materials have a lower density, higher specific strength and stiffness, and better damping qualities than traditionally used metals. The most common method of joining composite tubes to each other has been to use metal lugs at the joints and bonding of the composite tubes to the metal lugs. The metal lugs generally weigh more than the composite tubes to which they are bonded. If the metal lugs are reduced in size to reduce weight there must be a corresponding reduction in size of the composite tubes, with corresponding loss of strength of the frame. Since weight reduction and strength are of primary importance, the use of a combination of composite tubes metal lugs has not been entirely satisfactory.
Previous Jointed All Composite Frames
Several methods have been developed to produce an all-composite bicycle frame. One typical method involves secondarily joining precured composite tubes to each other in the desired frame configuration. Composite tubes are joined to each other by first cutting the tubes, trimming or mitering the tube ends and wrapping uncured composite materials around and between the ends and formed cuts of the tubes to be connected and then curing the composite materials, in place, to form a connection between the tubes. See U.S. Pat. No. 5,188,384 to van Raimdonck, U.S. Pat. Nos. 5,106,682 and 5,116,071 to Caffee, U.S. Pat. No. 4,900,048 to Derjinsky, and U.S. Pat. No. 5,019,312 to Bishop. The connection between composite tubes are generally solid and the resultant frame is not hollow throughout. The uncured composite material. used becomes a reinforcing material laid over and placed into and between the tube members. Parasitic material, which is generally unreinforced resin containing no fillers to control its viscosity and to reduce its density, is also used to insure acceptable cosmetic appearance of the assembled frames. The parasitic material is used to fill gaps and voids and then must be smoothed to give a desirable appearance to the frame. Frames made according to these known processes are not generally amenable to mass production, primarily due to the amount of manual labor required at each stage of the manufacturing process. Consequently, the frames are not commercially viable.
Another method for forming an all composite jointed frame uses lugs formed of composite materials using a bladder. In this method, disclosed in U.S. Pat. No. 5,624,519 to Nelson et al., bicycle lugs are formed by inserting preforms of stacked resin impregnated carbon fiber plies into respective halves of female tooling. A bladder is then placed over one of the preforms and the mold is closed. The bladder is then inflated to press the preforms against the tooling, and the mold heated to cure the resin to form the final cured lug. The preforms are sized so that each forms one half of the lug plus an overlapping portion that forms a lap edge. Accordingly, lugs manufactured by this process have dividing lines of overlapping cured composites. These dividing lines of overlapping composites provide weakness areas in the lugs and undesirable increased weight.
Trend Towards Elimination of Joints in All-Composite Frames
The joints in bicycle frames greatly influence the design, construction, and performance of the frames. Joints between frame members are the most frequent source of structural problems occurring in bicycle frames. This is because internal structural loads, are generally the greatest at the joints, and because it is difficult to bond the different materials at the joints. As a result, a number of proposals have been made to eliminate or greatly reduce the number of joints in the frames, through the greater use of composite materials. See U.S. Pat. No. 5,271,784 to Chen et al., U.S. Pat. No. 3,375,024 to Bowden, U.S. Pat. No. 3,833,242 to Thompson, U.S. Pat. No. 4,015,854 to Ramond, U.S. Pat. No. 4,230,332 to Porsche, U.S. Pat. No. 4,493,749 to Brezina, and U.S. Pat. No. 4,986,949 to Trimble.
Jointless All-Composite Bicycle Frames
High performance composite materials, such as carbon fiber with epoxy resin, have allowed the development of high performance bicycle frames made without any joints, or a reduced number of joints. In U.S. Pat. No. 4,828,781 to Duplesis, U.S. Pat. No. 5,544,907 to Lin et al. U.S. Pat. No. 5,368,804 to Hwang et al., U.S. Pat. No. 5,273,303 to Hornzee-Jones, and U.S. Pat. No. 4,986,949 to Trimble are disclosed methods for forming bicycle frames from carbon fiber/carbon composite materials without any joints, or a reduced number of joints. While these frames are successful to the large extent, there are many practical concerns, and manufacturing problems relating to the complexity of forming the entire bicycle frame in one step. In addition, the frames have a lapped construction, i.e., wherein two halves of overlapping materials form a lapped joint.
Three Dimensional vs. Two Dimensional
In the prior-art, parts that are simple three-dimensional shapes, often referred to as xe2x80x9ctwo-dimensionalxe2x80x9d because they can be defined by a two-dimensional plane rotated about an axis, such as tubes, or a two dimensional shaped traveled along a line, such as rectilinear solid have been made. As illustrated, for example in the U.S. Pat. No. 4,828,781, tubes in particular have been made without seams by various processes by wrapping resin impregnated fiber into tubes and curing the tubes under pressure applied externally or by an internal bladder. However, complex three-dimensional shapes, such as lugs, one piece frames, or other large frame sections, have been formed by overlapping fiber preforms to form a lapped joint because the methods for forming tubes and the like cannot be applied to complex three-dimensional shapes. This lapped joint not only introduces a potential weakness to the part, but requires additional material for the lap. In addition, this lap construction method may require additional reinforcing material in the lap area, which adds more weight to the final part.
Another problem with prior-art methods for forming complex three-dimensional parts is that a preform for each mold half must be dimensioned specifically to fit into the part cavity. This requires that each preform must be a large flat piece that when pressed into the mold cavity must distort and is likely to fold. These folds are a source of weakness and cracks in the final part and can lead to failure of the part. The folds can also result in resin pockets, pin-holes, and other visual and structural defects. The constraint to shape the preform for the mold also limits the possibility of designing the preforms specifically for performance and strength.
It is a principal object of the present invention to provide an all composite bicycle frame, wherein the entire frame or three dimensional components of the frame are lapless, thus avoiding the extra weight and strength reduction inherent in lapped construction.
It is another object of the invention to provide an all composite frame that has a low frame weight and high frame strength and stiffness.
Another object of the invention is to provide a method of producing a composite bicycle frame that includes making strong, hollow connector lugs suitable for interconnection of composite tubes.
Another object of the invention is to provide a lapless construction for complex three-dimensional parts, which allows easier formation of sockets for receiving other parts.
Another object of the invention is to provide a method for forming a complex three-dimensional composite part that is not constrained to specific preform shapes.
Another object of the invention is to provide a method for forming composite parts that has faster cycle times by eliminating the need for an operator to lay up preforms for the part in the mold, which allows the mold to be run hot and eliminates the time for lay up in the mold.
Another object of the invention is to provide a method for forming composite parts which reduces tool wear from material being caught between tool halves.
Another object of the invention is to provide a method for forming composite parts, wherein there is no constraint to dimension the preform for laying up in the mold.
Parts for three-dimensional composite bicycle components and frames of the invention are molded using matched female tooling conforming to the exterior shape of the part, with an internal pressurization system. A soluble or removable mandrel core conforming essentially to the interior of the part to be molded is formed from a foam material that remains soluble after the molding process. A sealed bladder, conforming generally to the shape of the desired lug to be produced, is placed around the soluble foam mandrel core. A bladder fitting is fitted to the sealed bladder such that the bladder fitting provides a path for air injection and bladder inflation. Layers of uncured composite material are usually laid in a piecewise fashion to create a covering of interleaving plies on the surface of the bladder. It is a relatively easy matter to place and hold the uncured layers of composite material on the bladder, since the soluble foam core functions as a support mandrel, and usually only one or two plies are laid up at a time.
The assembly comprising the soluble foam mandrel core, the sealed bladder, bladder inflation fitting, and the uncured composite material is placed in the female mold, with the bladder fitting attached to a suitable inflation fitting in the mold. The bladder is inflated through the bladder fitting to apply the necessary pressure to force the laid-up composite material against the cavity walls. The mold is then heated to the temperature necessary to cure the composite material. After the composite material has cured, the mold is opened, the cured part is removed, and the bladder fitting extracted from the cured part.
In a preferred embodiment, water or other suitable solvent solution is injected into the interior of the bladder to dissolve and wash out the soluble foam material. The water or solvent may be introduced through any suitable opening in the part, such as the opening left after removal of the bladder fitting. The soluble foam material of the mandrel core is used only for formation of the uncured preform of uncured resin composite, and it is not used in the actual molding process. Therefore, the strength and heat resistance of the foam is not critical during molding. Since the foam mandrel core does not have to withstand the pressure and temperature molding conditions, it may be made of materials that are structurally weak and unable to withstand elevated molding pressures and temperatures. This allows the use of materials that are completely unsuitable as removable molding cores used in some prior-art molding systems. Thus, a material, such as starch, functions as a suitable support for lay up of the plies and can be easily removed by dissolving with a readily available and non-toxic solvent, water.
The bladder is made of a heat resistant plastic that does not melt or react with or bond to the interior of the molded part. Thus, it can become separated therefrom and easily removed, along with any small residue of the foam core that may be within bladder. In the preferred embodiment of this invention, the bladder consists of a thin film, that comprises only a small volume of material. The film bladder can then be removed by pulling it out of even a small opening in the molded part. Additionally the thin film bladder itself can be a soluble plastic, preferably water soluble, such as polyvinyl alcohol (PVA) film. The use of easily soluble bladder films is advantageous where extremely small openings are used for inflation during molding and subsequent removal of the bladder. Soluble films are also used if molding of very complex features on the inside of the part might inhibit removal of the bladder. Such complexities might involve cocured metallic features inside the part.
After removal of the bladder and core, the completed part is trimmed if necessary. For example, the tube connector ends of a formed bicycle lug may be trimmed to have an entirely hollow, formed lug that will extend into and be bonded to hollow tubes to make up a fully composite bicycle frame. The formed bicycle lug may also form the receptacle into which the hollow tube can be inserted into and bonded to form an entirely composite bicycle frame. Other frame parts or complete frames may be also trimmed for attachment of metal or other composite components.
A summary of the steps for one of the preferred methods for producing bicycle frame components of this invention are listed below:
Step Description
1. Form a foam mandrel core which is used inside bladder for composite material lay up process.
2. Form and seal a bladder around the mandrel core and attach a bladder fitting to create the bladder/core/fitting assembly.
3. Place uncured fiber resin prepreg material on bladder/core/fitting assembly.
4. Place uncured fiber resin prepreg assembly containing bladder/core/fitting assembly into a female mold defining the outer contours of the final component.
5. Apply pressure to bladder through the bladder fitting to compact uncured fiber resin prepreg component while optionally applying vacuum in mold cavity to remove entrapped air while heating mold and prepreg to cure the plastic resin.
6. Remove cured part, which still contains the bladder/core/fitting assembly, from the female mold.
7. Remove the bladder fitting from the bladder/core/fitting assembly.
8. Introduce water into bladder/core, which is still inside component, to wash out the foam mandrel core from inside the bladder.
9. Remove the bladder from inside the cured part, by pulling it out of hole through which the inflation fitting extended, or any other suitable hole. In the case of the head lug and other lugs there may be multiple holes.
10. If required, trim the excess cured composite material. The part may be machined to create a bonding surface for attachment of additional parts or tubes.
11. Part is now ready for bonding to another part, such as a composite tube if the part is a frame lug.